ALBERT

Description: A new model for the provenance, depositional environment and tectonic setting of the Northern Belt of the Southern Uplands is presented. This turbiditic sandstone-dominated sequence was deposited in a sand-rich submarine fan environment, overlying sparse hemipelagic mudstones. The oldest sandstones are rich in juvenile ophiolitic material and record the first clastic input into the Southern Uplands basin. The bulk of the Northern Belt sedimentary sequence, however, is dominated by relatively quartzose sandstones derived from a Proterozoic continental/metamorphic source represented by the Midland Valley terrane of Scotland and Ireland. The quartzose-dominated succession was punctuated by the input of fresh volcanic detritus shed from an oceanic/continental island-arc situated to the W/NW of the Northern Belt basin, with sediment dispersal turning to the NE along the axis of the basin in Scotland. The tectonic setting of the Northern Belt basin remains uncertain. The complex provenance of the sandstones and recognition of major olistostrome units within the Northern Belt succession suggest that it was tectonically active. The onset of clastic deposition within the Southern Uplands terrane broadly corresponds to uplift and erosion of earlier obducted ophiolite in both Scotland and Ireland, possibly in response to collision of Cambrian–early Ordovician island-arc systems with the Laurentian continental margin. If this interpretation is correct, then the possibility arises that the Southern Uplands–Midland Valley terranes record the dismembering of this oceanic/continental island-arc complex within an overall transpressional regime.

Description: In the northern part of the Southern Uplands, restricted volumes of basic igneous rocks occur at or near the base of the Ordovician sedimentary strata. These rocks have previously been interpreted as ocean-floor tholeiites representative of the subducted Iapetus oceanic plate, preserved as tectonic slivers in a fore-arc accretionary prism. The alternative, back-arc basin model proposed for the Southern Uplands on sedimentological evidence raises questions over the origin of these rocks. New geochemical data and previously published data clearly indicate that the volcanic material does not have a simple single source. The oldest (Arenig) volcanic rocks from the Moffat Shale Group associated with the Leadhills Fault include alkaline within-plate basalts and tholeiitic lavas which possibly display geochemical characteristics of midocean ridge basalts. In the northernmost occurrence, alkaline and tholeiitic basalts contained within the Caradoc Marchburn Formation are both of within-plate ocean island affinity. To the south, in the Gabsnout Burn area, the Moffat Shale Group contains lenticular bodies of dolerite and basalt which have characteristics of island-arc to transitional basalts. This complex association of basaltic volcanic rocks is, at the present time, difficult to reconcile with either a simple fore-arc or back-arc setting for the Southern Uplands. However, the increasing arc-related chemical influence on basic rock geochemistry towards the southeast may tentatively be used in support of a southern arc-terrane, and as a result, a back-arc situation for the Southern Uplands basin. An alternative is that these volcanic rocks may represent the local basement to the basin and include remnants of an arc precursor to the Southern Uplands basin.

Description: Detrital clinopyroxenes occur abundantly in several greywacke formations in the Southern Uplands, both as single grains and within andesite lithoclasts. Compositionally, these pyroxenes fall into two main groups. Those from the oldest unit, the early Caradoc Galdenoch Formation, were derived from a simple tholeiitic volcanic arc. In contrast, the pyroxenes in the younger Portpatrick (late Caradoc) and Glenlee (Ashgill) Formations and the Gala Group (early Llandovery), show initial strongly calc-alkaline affinities, followed by a more complex calc-alkaline to transitional (calc-alkaline to tholeiitic) volcanic arc signature. The latter may be due to a mixture of calc-alkaline and tholeiitic components. A presently exposed source for the andesites has not yet been established, although the Ballantrae Complex is a potential candidate for a tholeiitic component.

Description: Regional lithogeochemical data from the Southern Uplands have been used to characterize a distinctive stratigraphy across this region. A suite of 840 point-counted and petrographically classified greywacke samples were used to establish chemical fingerprints for a series of greywacke-dominated lithostratigraphical units. These fingerprints were then used to evaluate a further 1455 greywacke samples collected throughout the Southern Uplands and Longford Down and enabled a series of strike-parallel geochemical tracts to be defined.Four principal geochemical groups are recognized, relating to cratonic- and volcanic-derived greywacke provenances and both carbonate-rich and hydrothermally altered greywackes. Volcanic-derived units display higher Ti, Fe, Mg, Ca, Na, Mn, Cr, Ga, Ni, Sr, V and Zn values than their cratonic counterparts, which, with the exception of the carbonate-rich Hawick Group, display higher Si, K, La, Nb, Rb, Th and Zr. Volcaniclastic greywackes display REE patterns dissimilar to typical post-Archean upper crust, but similar to their andesitic components, whereas cratonic groups have REE patterns close to that of upper crust. Systematic strike-parallel geochemical variation in the Southern Uplands is controlled by petrographical differences which directly reflect provenance, with individual lithostratigraphical units derived from variable mixtures of ophiolitic, calc-alkaline, acid-igneous, low-grade metamorphic and carbonate-rich detritus.

Description: Prominent aeromagnetic anomalies around Ballantrae in southwest Scotland have long been associated with extensive outcrops of basic/ultrabasic igneous rocks in the Midland Valley terrane. However, recent reprocessing of data from a 1978 detailed aeromagnetic survey has demonstrated excellent correlation between discrete aeromagnetic features and certain magnetic greywacke formations within the early Caradoc Tappins Group. This heterogeneous unit forms the oldest and most northwesterly tectonostratigraphic tract in the Southern Uplands terrane and is characterized by relatively high detrital magnetite(and serpentinite) content and elevated values for Cr and Ni. This suggests original close sedimentary links with an ophiolitic terrane not unlike the presently adjacent Ballantrae Complex (or its extensive along-strike correlatives), a consideration which has important implications for terrane reconstructions in this region.